Integrated circuit chips use power supplies to convert an input voltage from a power source to an output voltage across an output load. Typically, such power supplies are regulated in order to maintain a constant output voltage across the output load even if the output load changes and/or the input voltage varies.
A switched mode power supply utilises a switching arrangement between the power source and the output load. This switching arrangement continually switches ON and OFF. In the ON state, the power source is connected to the output load. In the OFF state, the power source is not connected to the output load. The duty cycle (proportion of ON time) is selected in order to achieve the desired output voltage. Excess energy is stored in the circuit until it is released to the output load.
FIG. 1 illustrates a switched mode power supply. This switched mode power supply supplies current from a power source connected to VSOURCE to a plurality of output rails 102, 103, 104 and 105 in order to power output loads on those output rails. A switching arrangement controls the connection of the power source to the remainder of the power supply. This power supply is a single-inductor-multiple-output (SIMO) power supply. It uses inductor 106 to store energy when the power source is connected to the remainder of the power supply. The particular arrangement illustrated in FIG. 1 is a buck-boost converter. In the buck mode, the voltage output to an output rail is less than the source voltage VSOURCE. In this mode, switch 109 remains open and switches 107 and 108 continually switch ON and OFF. In the boost mode, the voltage output to an output rail is greater than the source voltage VSOURCE. In this mode, switch 107 remains closed, switch 108 remains open and switch 109 continually switches ON and OFF. In the buck-boost mode, the voltage output to an output rail may be greater or less than the source voltage VSOURCE, and all three switches switch ON and OFF.
Switch 107 is typically a PMOS transistor. When the source voltage VSOURCE is sufficiently high, the PMOS transistor acts as a very efficient switch. However, if the gate source voltage of the PMOS transistor is close to a threshold voltage (˜0.8V) then the transistor becomes very resistive. It does switch current from the power source through to the remainder of the power supply in this region, but much of that current is dissipated as heat. Thus, the efficiency of the power supply is degraded.
There is increasing market demand for lower power devices. Thus, there is a need for a switched mode power supply that operates more efficiently at low source voltages.